CHAPTER XII. 



SUMMARY 



THE investigation now concluded is based on the fact 

 that the characteristics of any population that is in 

 harmony with its environment, may remain statistically 

 identical during successive generations. This is true 

 for every characteristic whether it be affected to a great 

 degree by a natural selection, or only so slightly as to 

 be practically independent of it. It was easy to see 

 in a vague way, that an equation admits of being based 

 on this fact ; that the equation might serve to suggest 

 a theory of descent, and that no theory of descent that 

 failed to satisfy it could possibly be true. 



A large part of the book is occupied with preparations 

 for putting this equation into a working form. Obstacles 

 in the way of doing so, which I need not recapitulate, 

 appeared on every side ; they had to be confronted in 

 turns, and then to be either evaded or overcome. The 

 final result was that the higher methods of statistics, 

 which consist in applications of the law of Frequency 

 of Error, were found eminently suitable for expressing 



